The regulation of secretion of ADH in an infant with persistent hypernatremia will be investigated. Plasma ADH levels and urine osmolality will be determined following volume expansion and contraction and in the face of rising serum osmolality. Endogenous ADH stores and the renal response to exogenous ADH will be evaluated. The data will add to knowledge of the physiology of ADH secretion and may guide subsequent therapy in this infant.